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A fish's ear may hold the key in calculating when sex change occurs in reef fish, an Australian marine scientist says.

In the latest edition of the Royal Society's Biology Letters journal, doctoral student Stefan Walker has found that the markings on the otolith, or ear stone, act like rings on a tree in showing at what age an individual fish changes sex.

Walker, of the School of Marine and Tropical Studies at James Cook University says knowledge of when and how sex change occurs is important for aquaculture as it enables the industry to maximise fish reproduction.

He says in aquaculture the larger specimens, which are predominantly male, tend to be removed for consumption.

"We need to know when and how they change sex, so we know how males can be replenished to ensure reproduction is maximised," Walker says.